This invention relates to a method of, and apparatus for use in, magnetic resonance imaging; and more particularly, to contrast agent enhanced magnetic resonance arteriography for examining, detecting, diagnosing, and treating arterial diseases and injuries in arteries in the lower extremities, including defining anatomic features relevant to performing arterial surgery for atherosclerotic disease.
Arterial diseases and injuries are common and often have severe consequences including death. Imaging arteries serves to screen, detect and characterize arterial disease before these consequences occur. It also serves to define anatomic features which may provide assistance when performing surgery for atherosclerosis.
Atherosclerosis is a major problem in the aged population, particularly those in developed countries. This disease tends to be progressive in a considerable number of instances and may result in significant morbidity; and, in instances of severe atherosclerotic disease, lower limb amputations and/or mortality.
Early detection of atherosclerosis may lead to a decrease incidence of complications by allowing earlier treatment of arterial stenoses or atherosclerotic disease. Of the conventional techniques, catheter arteriography is the xe2x80x9cgold standardxe2x80x9d for delineation of the arterial tree of the lower extremities. This technique involves inserting a catheter into the artery of interest (the artery under study) and injecting radiographic contrast, for example, an iodinated contrast, while acquiring radiographs of the artery. Radiographs are commonly referred to as X-rays. The contrast remains in the arteries for a few seconds during which time the arteries appear distinct from both the veins and background tissue in the radiographs.
Although a catheter-based contrast arteriography technique generally provides high quality arterial images, there is a risk of arterial injury or damage by the catheter and its insertion. There may be thrombosis, dissection, embolization, perforation or other injury to the artery itself. Furthermore, such a technique may result in a stroke, loss of a limb, infarction or other injury to the tissue supplied by the artery. In addition, hemorrhage at the catheter insertion or perforation sites may require blood transfusions. Moreover, kidney failure and brain injury may result from the toxic effects of the X-ray contrast.
Because of its invasive nature, cost and complication rate, catheter arteriography is typically not a suitable screening technique for detection of stenoses or atherosclerotic disease. Rather, catheter arteriography is most often used prior to angioplasty or surgical reconstructive proceduresxe2x80x94that is, after detection and diagnosis of a stenoses or atherosclerotic disease.
Further, although catheter arteriography is highly accurate under ideal conditions, it often fails to demonstrate distal vessels suitable for bypass in more than half of the patients with severe disease. Additionally, overlapping cortical bone can make interpretation of overlying vessels difficult.
There are several conventional non-invasive tests for diagnosis of peripheral vascular disease including B-mode and color flow Doppler sonography. Sonography provides indirect information of stenoses based on waveforms and velocity measurements. These type of tests require a skilled and experienced examiner to maintain acceptable accuracy and are often problematic in obese individuals. Although imaging from the groin vessels to the level of the mid popliteal artery is possible in many patients, arterial information below the mid popliteal artery is frequently inaccurate. In addition, because sonography provides indirect information of the arterial characteristics, sonography cannot image stenoses directly in the majority of cases. As such, estimation of the degree of luminal stenosis relies on velocity measurements which can often be inaccurate.
Magnetic resonance angiography has several advantages over conventional or catheter arteriography. Magnetic resonance angiography does not use ionizing radiation, and does not require arterial catheterization and sometimes can be performed without contrast agent enhancement. Even when performed using a contrast agent (e.g., gadolinium), magnetic resonance angiography contrast is safer than iodinated contrast arteriography, and it infrequently causes the patient discomfort. In addition, the cost of an magnetic resonance arteriogram is less than a catheter arteriogram. Further, because of its sensitivity to slow flow MRA may be more sensitive in vessels with proximal stenoses.
However, there are several impediments or limitations to use of magnetic resonance angiography as a satisfactory screening tool for imaging of the lower limb vasculature. These impediments or limitations have been documented in, for example, Owen, et al., Magnetic Resonance Imaging of Angiographically Occult Runoff Vessels in Peripheral Arterial Occlusive Disease, N Eng. J Med 1992, 326:157-1581; Owen et al., Symptomatic peripheral vascular disease: Selection of imaging parameters and clinical evaluation with MR angiography, Radiology 1993, 187:627-635; Yucel et al., Atherosclerotic occlusive disease of the lower extremity: prospective evaluation with two-dimensional time-of-flight MR angiography, Radiology 1993, 187:635-641; and Borrello, MR Angiography versus conventional X-ray angiography in the lower extremities: Everyone wins, Radiology 1993, 187:615-617).
One of the limitations to employing contrast enhanced magnetic resonance angiography for imaging of the lower limb vasculature has been the lack of a suitable coil for imaging of a sufficiently large field-of-view which encompasses the lower extremities. Another potentially more serious problem involves limitations of magnet imaging region. In this regard, the length of the magnet imaging region for conventional magnet resonance apparatus is insufficient to cover the entire anatomical region-of-interest in one acquisition. This can be as much as 120 centimeters in tall patients and even in small patients an imaging field encompassing approximately 90 centimeters is necessary to compete with arteriography which images from above the aortic bifurcation downwards.
Because of the reliance of traditional magnetic resonance arteriography methods on time-of-flight effects, imaging orthogonal to the plane of the vessel necessitates image acquisition in the axial plane. As such, it is necessary to reposition the patient and perform an additional localizer for successive locations down the legxe2x80x94all of which is time-consuming. In order to maintain spatial resolution, thin slices must be obtained giving poor spatial xe2x80x9ccoveragexe2x80x9d per unit time. Under these circumstances, imaging times may be in excess of one hour and frequently 2 hours for comprehensive imaging of the lower limb vessels. Although acquisition of the 2-D time-of-flight images in the coronal plane would significantly reduce imaging time and therefore cost, imaging in this plane would result in saturation of the flowing blood and non-diagnostic studies.
One solution to the problem of in-plane saturation has been to employ contrast enhanced magnetic resonance arteriography to overcome saturation effects. See, e.g., U.S. Pat. Nos. 5,417,213; 5,553,619; and 5,579,767 (the contents of each are hereby incorporated by reference). Using this technique, acquisition of the central lines of k-space which govern image contrast are acquired during peak arterial enhancement by carefully timing the injection of the contrast agent with the collection of image data which is representative of the center of k-space. A technique of more precisely acquiring the central lines of k-space during peak arterial enhancement by detecting arrival of the gadolinium bolus and initiating data acquisition of data which is representative of the central lines of k-space is described in U.S. Pat. No. 5,590,654. These technique have been shown to be highly accurate compared to arteriography or surgical inspection in the evaluation of abdominal aortic aneurysms, thoracic aorta, renal and mesenteric arteries, and aorta and iliac vessels.
In spite of the high quality images of the abdominal aortic aneurysms, thoracic aorta, renal and mesenteric arteries, and aorta and iliac vessels which have been consistently obtained using contrast enhanced magnetic resonance arteriography, there still remains several problems with using such imaging techniques to evaluate arteries in lower extremities. By using a gadolinium contrast agent, high signal-to-noise images are generated within the body coil. Although this overcomes the problems of both in-plane saturation effects and the necessity for expensive and as yet experimental surface coils, the imaging volume is limited by the largest field-of-view. The field of view, however, is governed by the physical dimensions of the body magnet or coil (typically 48 centimeters or less). Even if this large field-of-view could be used, the increased matrix size necessary to maintain resolution would increase examination time (increased number of phase-encoding steps) and increase echo time (increased frequency-encoding steps), both of which are undesirable. Additionally, the signal-to-noise from the top and bottom ends of the imaging volume would likely be inadequate for diagnostic purposes; and even if adequate images were obtained over the entire imaging volume, the anatomical coverage would still be insufficient for adequate evaluation of the lower extremities.
Although a second sequence centered at a lower level would provide the xe2x80x9cmissingxe2x80x9d diagnostic information, this presents additional concerns of the time dependence of the arterial signal which would likely diminish to a level below that necessary for diagnostic imaging in relation to the time necessary for re-localization and re-prescription of another sequence at another, lower level. This time delay in collecting image data for the second, lower level would be of such a magnitude that enhancement of lower limb veins would complicate interpretation of images and probably render lower extremity arterial imaging non-diagnostic.
As a result, there exists a need for an improved apparatus and method for magnetic resonance arteriography which provides an image of the arteries distinct from the veins and which overcomes the limitations of other techniques. There exists a need for an apparatus and technique which allows preferential imaging of the lower limb arterial tree in a sufficiently short time period to allow imaging without significant venous overlap and without the complications often observed or experienced with catheter arteriography.
In addition, there exists a need for a contrast (e.g., gadolinium) enhanced magnetic resonance arteriography technique which provides essential and accurate anatomic information for arterial reconstructive surgery and which is devoid of contrast-related renal toxicity or catheterization-related complications attending catheter arteriography.
In one principal aspect, the present invention is a method of imaging arteries in a patient using a magnetic resonance imaging system having an imaging coil and an administered contrast agent. The arteries include a first artery which is located in a first image volume and a second-artery in a second image volume.
The method of this aspect of the invention includes positioning the patient in a first location in the imaging coil and then collecting image data of the first image volume including collecting image data which is representative of a center of k-space while a concentration of the administered contrast agent in the first artery is substantially higher than a concentration of the contrast agent in veins and background tissue adjacent to the first artery and while the patient is in the first location. The method further includes re-positioning the patient in a second location in the imaging coil and, then collecting image data of a second image volume, including collecting image data which is representative of a center of k-space while a concentration of the administered contrast agent in the second artery is substantially higher than a concentration of the contrast agent in veins and background tissue adjacent to the second artery and while the patient is in the second location.
The method may further include constructing an image of the first and second arteries using the image data of the first and second image volumes.
In another principal aspect, the present invention is a magnetic resonance imaging system for imaging arteries in a patient using an injected contrast agent of substantially one injection. The arteries include a first artery which is located in a first image volume and a second artery which is located in a second image volume. The system includes an imaging coil and a platform for supporting the patient in a substantially horizontal posture, the platform being moveable with respect to the imaging coil and between a plurality of locations along a longitudinal axis of the platform. The system also includes imaging means for collecting image data of the first image volume while the platform is positioned at a first location and image data of the second image volume while the platform is positioned at a second location.
The image data of the first image volume includes image data which is representative of a center of k-space while a concentration of contrast agent in the first artery is substantially higher than a concentration of contrast agent in veins and background tissue adjacent to the first artery. The image data of the second image volume includes image data which is representative of a center of k-space while a concentration of contrast agent in the second artery is substantially higher than a concentration of contrast agent in veins and background tissue adjacent to the second artery.
In one embodiment, the system further includes means for moving the platform between a plurality of discrete locations along the longitudinal axis.
In another embodiment, the imaging means generates a data collection complete signal in response to completing collection of the image data of the first image volume and wherein the platform, in response to the data collection complete signal, automatically moves from the first location to the second location. In this embodiment, the platform may generate a move complete signal in response to arriving at the second location and wherein the imaging means automatically collects the image data of the second image volume in response to the move complete signal.
In another embodiment, the system includes monitoring means for monitoring a concentration of the injected magnetic resonance contrast agent in a region of interest and wherein the imaging means collects image data of the first image data volume in response to an image acquisition signal. In this embodiment, the system may also include means for generating the image acquisition signal when the magnetic resonance contrast agent in the region of interest is above a predetermined concentration.
In yet another principal aspect, the present invention is a magnetic resonance imaging system for imaging arteries in a patient using an injected contrast agent of substantially one injection. The arteries include a first artery which is located in a first image volume, a second artery which is located in a second image volume, and a third artery which is located in a third image volume. The system includes an imaging coil and a platform for supporting the patient, the platform being moveable along a horizontal axis of the platform between a first location on the longitudinal axis, a second location on the longitudinal axis, and a third location on the longitudinal axis.
The system of this aspect of the invention also includes an imaging unit, electrically coupled to the imaging coil, the imaging unit collects image data of the first image volume while the platform is positioned in the first location and image data of the second image volume while the platform is positioned in the second location, and image data of the third image volume while the platform is positioned in the third location. The image data of the first image volume includes image data which is representative of a center of k-space while a concentration of contrast agent in the first artery is substantially higher than a concentration of contrast agent in veins and background tissue adjacent to the first artery; the image data of the second image volume includes image data which is representative of a center of k-space while a concentration of contrast agent in the second artery is substantially higher than a concentration of contrast agent in veins and background tissue adjacent to the second artery; and the image data of the third image volume includes image data which is representative of a center of k-space while a concentration of contrast agent in the third artery is substantially higher than a concentration of contrast agent in veins and background tissue adjacent to the third artery.
In a preferred embodiment, the system includes a platform moving means for automatically moving the platform between the first, second and third locations. In this embodiment, the imaging unit generates a data collection complete signal in response to completing collection of the image data of the first image volume, and wherein the platform moving means, in response to the data collection complete signal, automatically moves the platform from the first location to the second location.
In another embodiment, the imaging unit generates a data collection complete signal in response to completing collection of the image data of the second image volume, and wherein the platform moving means, in response to the data collection complete signal, automatically moves the platform from the second location to the third location, wherein the platform moving means generates a move complete signal in response to arriving at the third location and wherein the imaging unit automatically collects the image data of the third image volume in response to the move complete signal.
In another embodiment, the imaging unit constructs an image of the first, second and third arteries using the image data of the first, second and third image volumes.
In yet another embodiment, the first image volume overlaps the second image volume by less than about 25% and the second image volume overlaps the third image volume by less than about 25%.